The present invention relates generally to taps for cutting screw threads into stock material, and more particularly relates to tandem cutting taps having multiple sections for cutting threads of differing thread characteristics.
Taps are used to cut screw threads into the inner wall of a formed bore in stock material. Tandem taps are a special type of tap that includes two or more threaded sections for creating internal threads. Typically, tandem taps are utilized to create threads of the same pitch, but at different outside diameters that share a common axis. Tandem taps can also be utilized to facilitate tapping of difficult thread forms, such as Acme threads, by combining graduated thread forms from one section of the tap to the next.
While tandem taps have the advantage of cutting two sets of threads with one tapping or work operation (as opposed to two work operations) and automatic axial alignment of two different sets of threads that are being cut, tandem taps of the prior art have had the drawback of frequently experiencing failure or tap breakage. Tandem taps frequently break when chip material builds up in the transition area between the front and the back sections of the tandem tap.
It is an objective of the present invention to reduce the likelihood of tap breakage in tandem taps due to chip material buildup in the transition area between front and back sections of the tap.
In accordance with these and other objectives, the invention comprises tandem tap for cutting internal threads on an inner wall of a bore that simultaneously moves chip material in two different directions when cutting. The tandem tap comprises an elongate body for rotation about an axis extending between a workable end and a tip end. The elongate body has a first cutting section toward the tip end and a second cutting section between the first cutting section and the workable end. The workable end is adapted to be worked for rotating the tandem tap. The first and second cutting sections include external cutting threads of different thread characteristics wherein the first and second cutting sections are adapted to cuts threads having different characteristics. The tap includes at least one external first flute for chip clearance that interrupts the external threading on the first cutting section. The first flute extends axially to the tip end. When the first cutting section is cutting threads, the chips produced thereby are moved in a forward direction. The tap also includes at least one spiral flute interrupting the external threading on the second cutting section. The spiral flute follows a helical path leading toward the workable end. When the second cutting section is cutting threads, the chips produced thereby are moved in a rearward direction opposite the forward direction.
The present invention is also directed toward an improved method for cutting threads into an inner wall of a bore by rotating a tandem tap inside the bore. The method comprises: cutting a first screw threading into the inner wall, thereby producing a first set of chips and simultaneously cutting a second threading into the inner wall in axial spaced relation to the first threading. The second screw threading is of a different thread characteristic than the first screw threading, thereby producing a second set of chips. The method further comprises moving the first set of chips in a forward direction for clearing the first set of chips; and simultaneously moving the second set of chips in a rearward direction opposite the forward direction for clearing the second set of chips.
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.